This invention relates to a method of depositing thin films, particularly thin films such as metallic films, in the fabrication of integrated circuits.
Present trends in the formation of vacuum deposited thin metallic film commonly use chemical etching in the presence of etch-resistant masking layers to provide the selected pattern. This is the traditional photo engraving or photo lithographic etching technique. However, with the continued miniaturization of semiconductor integrated circuits to achieve greater component density and smaller units in large scale integrated circuitry, the art is rapidly approaching a point where such photo lithographic etching of deposited film may be impractical for providing the minute resolution required for the fine line work of metallization in such large scale integrated circuitry.
An alternative method for forming such metallization is commonly denoted by the term "expendable mask method," "lift-off method," or "stencil method." U.S. Pat. No. 3,873,361, issued Mar. 25, 1975 to Havas et al, entitled "A Method of Depositing Thin Film Utilizing a Lift-off Mask" and assigned to the present assignee discloses a lift-off method for depositing thin films which avoids the "edge-tearing" problem and is suitable for use were the lateral widths of the spacing between adjacent deposited metallic lines is of the order of 0.05 to 0.25 mils. The disclosed method includes the use of an organic polymeric material deposited on the integrated circuit substrate and an overlying layer of an inorganic material, preferably metal, having openings in the selected pattern. Openings are formed in the polymeric material by reactive sputter etching utilizing the metallic mask as a barrier. The openings in the polymeric layer are aligned with and laterally wider than the corresponding openings in the metallic masking layer as a consequence of the reactive sputter etching step. Thus, the edges of the openings in the metallic masking layer overhang the edges of the openings in the underlying polymeric layer. The thin film to be deposited is then applied over the structure and on the surface of the substrate exposed by the openings in the polymeric material. When the polymeric material is removed by application of solvent, the metallic masking layer and the thin film above the masking layer "lift off" to leave the thin film deposits in the selected pattern on the substrate without "edge tearing" of the desired deposited thin film as the unwanted portions of the thin film are lifted off.
With the metallic overlay process, i.e., the use of the metallic (opaque) reactive sputter etching mask, alignment of overlying patterned layers with respect to the underlying patterned substrate can be difficult. One reason for this is that the typography of the underlying substrate is made quasi-planar by the spun-on polymeric layer upon which the metal mask is evaporated. Another reason is that the opacity of the metal makes it troublesome optically to distinguish alignment marks on the substrate. This alignment problem is alleviated in accordance with the teachings of the aforementioned patent by reserving two alignment areas at opposite ends of the wafer substrate which are left unmetallized during metal mask evaporation. An undesirable consequence is that the alignment areas are not available to be used for the production of active circuit components and reduces circuit yield on each wafer. Additionally, the use of the evaporated metal reactive sputter etching masking layer requires the use of a relatively expensive and time consuming evaporation step and a subsequent chemical etching step to pattern the evaporated layer.